This invention relates to a flow-control valve assembly and, in one aspect, is particularly concerned with a flow-control valve assembly for controlling the flow of a field-responsive fluid bidirectionally. This invention also relates to dampers incorporating such valve assemblies and to the use of such dampers.
Examples of field-responsive fluids (hereinafter simply called FR fluids), are electro-rheological fluids (hereinafter simply called ER fluids), whose rheology can be changed by an electric field, and magneto-rheological fluids (hereinafter simply called MR fluids), whose rheology can be changed by a magnetic field. ER fluids exhibit a characteristic change in viscosity or apparent viscosity when an electric field is applied; and in the case of MR fluids, a similar effect is observed when a magnetic field is applied.
For effecting flow control in ER fluids, it is commonly the practice to pass such ER fluid along an extended path between electrodes and to apply an electric field to the ER fluid so as to control its resistance to flow and therefore the pressure drop across the restrictor thereby defined. Similarly, flow control of MR fluids can be controlled by applying a magnetic field. The rate at which energy can be dissipated through the ER or MR effect is inter alia proportional to the volume of ER or MR fluid in the appropriate electric or magnetic field. In practice, this often means that devices using such fluids are too large for certain applications or too large to be economically feasible, both in terms of the volume of FR required and the large power demands necessary, particularly where high pressure drops are required.
WO94/21938 discloses a flow-control valve suitable for use with ER fluids wherein a first flow restrictor including a resilient diaphragm is disposed in a flow path between an inlet port and an outlet port and a second flow restrictor is disposed in the flow path upstream of the first flow restrictor so as to affect the pressure drop across the diaphragm. The second flow restrictor includes electrodes for applying an electric field as it passes through the second flow restrictor, to enable control of the resistance to flow of the ER fluid therethrough and thereby to enable control of the pressure drop across the diaphragm.
It has been found in practice that the control valve disclosed in WO94/21938 can be difficult to control and tends to be either fully closed or fully open, with little repeatable control between these two states. Additionally, the majority of the pressure drop across the diaphragm is found to be due to pressure drops throughout the entire flow path, rather than as a result of fluid flow through the ER valve section. This can lead to difficulties in ensuring a controlled and well-defined pressure drop across the diaphragm.
For use in a damper, it is important that this flow-control valve does not remain completely closed when the damper experiences a large force, since there would be no damping and the lack of pressure relief may give rise to a dangerous situation.
It is an object of the first aspect of the present invention to provide a field-responsive-fluid-control valve assembly which obviates or mitigates these problems.
According to said first aspect of the present invention, there is provided a flow-control valve assembly for a field-responsive (FR) fluid, said valve assembly comprising:
an inlet port;
an outlet port;
a flow path for FR fluid extending between the inlet and outlet ports;
a flow-control valve disposed in the flow path between the ports, said flow-control valve including a valve member which is moveable between a first position in which the valve is open, and a second position in which the valve is closed;
a flow restrictor disposed in the flow path upstream of the flow-control valve, said flow restrictor including means for applying a field to the FR fluid as it passes through the flow restrictor to enable control of the resistance to flow of the FR fluid therethrough, so as to affect a pressure drop across the valve member, and thereby to control movement of the valve member between its first and second positions; and
a bypass passage defining a pressure-relief means, which can permit flow between inlet and outlet ports when the valve member is in the second position, said bypass passage being disposed downstream of the flow restrictor.
When an MR fluid is used, the flow rate of MR fluid passing through the valve assembly can be controlled by a magnetic field that is controlled by the applied electric current. When an ER fluid is used, the flow rate of ER fluid passing through the valve assembly can be controlled by an electric field that is controlled by the applied voltage. A combination of ER and MR fluids may be used, in which case control will be provided by both the voltage and current signals.
The means for applying an electric field to the ER fluid may be of the type described in WO94/21938. The means for applying a magnetic field to the MR fluid may include a permanent magnet or an electromagnet. In the case of an electromagnet, a field of varying intensity can be applied by adjusting the current passing through the electromagnet.
Preferably, the valve member is resiliently biased into its first position and, more preferably is a resilient diaphragm, for example a diaphragm of the type described in WO94/21938.
The flow-control valve may additionally comprise means for varying the resistance to deformation of the diaphragm. Preferably said means comprises a diaphragm support which is shaped so that, in use, the effective diameter of the diaphragm is changed, preferably reduced, as it is deformed.
The size of the bypass passage (cross-sectional area and length) determines the maximum energy that the flow-control valve assembly can dissipate for a given flow-control valve. The bypass passage may incorporate a pressure-relief valve which may be a cantilevered spring flap. The force on the pressure-relief valve is determined inter alia by the pressure drop across the valve member as it closes. Advantageously, the threshold opening pressure of the pressure-relief valve and the size of the bypass passage can be varied from one application to another, in order to provide different pressure-relief profiles. For example, in the case of a cantilevered spring flap, the strength of the spring may be such that the flap gradually opens as the valve member closes. Alternatively, a stronger spring may permit the valve member to close completely before the flap opens.
According to a second aspect of the present invention, there is provided a damper comprising a cylinder in which a piston with piston rod is slidable, an FR fluid-flow passage interconnecting opposite sides of the piston, and a flow-control valve assembly controlling flow of FR fluid through said fluid-flow passage, wherein the flow-control valve assembly is in accordance with said first aspect of the present invention.
Preferably said piston incorporates said flow-control valve assembly.
Said damper may include first and second flow-control valve assemblies adjacently disposed and orientated so that they respectively control flow of FR fluid through said fluid-flow passage in opposite directions, wherein at least one of said flow-control valve assemblies is in accordance with said first aspect of the present invention. Additionally, said flow-control valve assemblies may each include a one-way valve disposed so as to permit flow into the FR fluid-flow passage.
Preferably said flow-control valve assemblies share a common valve member. Most preferably, the common valve member is a resilient diaphragm which is moveable between a first position in which both flow-control valves are open, and second and third positions in which only th e first flow-control valve is open and only the second flow-control valve is open, respectively.
Advantageously, the flow-control valve assemblies can be constructed to have different flow-control characteristics and/or can be controlled, in order to present different damping effects on the compression and extension strokes of the piston.
According to a third aspect of the present invention, there is provided a fluid-flow control device, comprising a body having an FR fluid-flow passage therein, and control means to apply a field across the passage so as to vary the resistance to flow of the FR fluid through the passage, wherein the control means is arranged to apply a variable field pulsing between relatively high and low states.
Preferably said high and low states are on and off states, ie the power supply is digital. Preferably said control means incorporates means to allow the width of the pulses to be varied.
Advantageously, no control is required other than for the duration of the pulses, since the magnitude and smoothness of the pulses are not critical, making the power supply simple and inexpensive to produce.
The pulsed field may be provided by an alternating or pulsed electric voltage to control an ER fluid or an alternating or pulsed electric current to control an MR fluid.
Preferably, a flow-control valve assembly according to said first aspect of the present invention is controlled by a pulsed field in accordance with said third aspect of the present invention. Since the damping force is determined by the width of the pulses, there is no need to control the valve member accurately during transition between its first and second positions.
Advantageously, such a pulsed field can be used to enable the valve member to oscillate in use between its first and second positions. By varying the width of the pulses, any value of damping force can be developed between the minimum and maximum of the flow-control device.
Preferably, a damper according to said second aspect of the present invention uses an ER fluid and is controlled by a pulsed voltage in accordance with said third aspect of the present invention.
Advantageously, a much smaller flow-control valve can be used, reducing power consumption significantly. In addition, weaker FR fluids may be used, since the FR fluid is required only to operate the flow-control valve, not to provide the total damping force. The high energy loss dependent on the length and cross-sectional area of the bypass passage is independent of variations in temperature. The overall effect is that the damper mimics a manually controlled or solenoid-controlled dual-orifice damper, but with a significantly faster response. Thus it is to be understood that the damping force can be altered within a single stroke of the damper.
The width of the pulses may be determined by the force experienced by the damper, so that for a small force the xe2x80x9conxe2x80x9d pulses are short and for a large force they are long. Alternatively, the width of the pulses may be linked to the length of travel of the piston in the cylinder, with longer xe2x80x9conxe2x80x9d pulses provided for longer movements of the piston. In either case, it is possible to balance a sufficient degree of damping with smooth motion of the piston.
A damper according to the second aspect of the present invention may be used in an agricultural vehicle seat assembly. Further uses for such a damper include prosthetic limbs, bicycle or motorcycle suspension units, lateral or vertical rail rolling stock damping, primary automotive suspensions, vehicle cab, ambulance and motor vehicle isolation, washing machine drum rotation damping, high speed centrifuge balancing and exercise equipment such as rowing machines and multigyms.